This invention relates generally to a screw thread structure and in particular to an improvement of a screw thread structure disclosed in U.S. Pat. No. 4,799,844.
In the art of percussion drilling, such as for mining, quarrying water well drilling, and the like, various components of a percussion drilling tool are frequently coupled together by threaded screw structures. In these coupled drilling components, the thread design is of considerable importance since failure often occurs in the screw structure. Whenever failure in the screw structure does occur, the initial crack usually initiates at the thread root. This is due to the high stress concentrations located at the root of the thread when the screw structure is subject to severe loading.
Traditionally, the root portion has been configured as a portion of a circle, tangentially adjoining the two flanks of successive thread turns. Generally speaking, the stress concentration along the thread root is an inverse function of the radius of that circle, i.e., the larger the radius of the circle, the lower the stress concentration. However, when the size of the radius surpasses a maximum allowable value, the stress concentrations at the ends of the root, where the flanks of the adjacent thread turns tangentially adjoin the root, become very high and thus provide starting points for crack propagation. Alteratively, when the size of the radius defining the circular root curvature decreases below a minimum allowable value, the stress concentration becomes very high at the bottom of the root. Thus, circular root curvatures are confined to radii sizes falling between certain maximum and minimum limits, depending on such factors as thread pitch and the like.
An improved screw structure has been disclosed in U.S. Pat. No. 4,799,844 wherein the screw structure of each of the threadedly interconnected drilling components includes a root portion configured as a portion of an ellipse. That elliptical root curvature provides a larger equivalent radius at the thread root which results in a decreased stress concentration along the root. Notwithstanding the benefits attainable by that screw structure, room for improvement remains, especially insofar as further relieving stress concentrations along the root.